Conventionally, if heat transfer tubes are used to recover heat from a molten slag directly, heat cannot be recovered in a continuous manner because lump of solidified slag adheres to the heat transfer tubes. In order to address such problem, there has been proposed a method for recovering heat after once cooling as well as solidifying the slag with an air or water spray, such heat being actually recovered at a temperature not higher than a solidification temperature (e.g., patent documents 1 and 2). However, such methods can not utilize the high exergy of high-temperature slag effectively.
Further, there have also been proposed: a method for recovering heat from high-temperature gas released when spraying gas such as air on a molten slag (e.g., patent document 3); and a method for recovering heat from a water vapor generated when mixing a molten slag with cold or warm water (e.g., patent document 4). However, these methods have a fault that large heat recovery device is required because heat transfer coefficient between gas and heat transfer tubes is low.